1. Field of the Invention
The present invention relates to a process for refining molten pig iron and steel, which process combines a converter refining step and a secondary refining step so as to remarkably enhance the all-round economy of the process.
2. Description of Prior Arts
In recent years the most predominant process for iron and steel making has been the blast furnace-converter process, and the principal refining ability attributed to the converter in this process is removal of carbon and phosphorus from liquid blast-furnace iron.
According to the conventional converter process which has been commonly adopted for steel making, the so-called catch-carbon method is used, which method comprises varying the predetermined blown-off carbon [C] content in correspondence to the predetermined carbon [C] content to be achieved in the final product by the process and changing the oxygen blowing condition accordingly.
The variation in the blown-off carbon content from charge to charge causes necessarily considerable fluctuation of (FeO) content in the slag, which in turn produces substantial influences on the phosphorus removing ability of the slag. For this reason, such an oxygen blowing method has been conventionally practised as to increase the (FeO) content in the slag so as to obtain a blown-off phosphorus [P] content which satisfies the standard specification of the phosphorus content in the final product. However, such changes in the operational conditions from charge to charge and the operation under the high-(FeO) slag as above inevitably result in a metallurgical operation susceptible to substantial fluctuations in various operational conditions, thus lowering the reproductibility of desired operational conditions, and causing in the last an unstable operation and an increased operation cost.
In order to eliminate the above difficulties and disadvantages, the so-called dynamic control method has been developed as an effective means for controlling the conventional operation susceptible to the substantial fluctuations. According to this method, the bath composition and temperature are measured in the course of oxygen blowing and the blowing trend is revised and adjusted on the basis of the intermediate measurements. This method, however, has been found in practical operations to be unable to maintain the simultaneous achievement of a desired blown-off carbon [C] content and a desired blown-off temperature at an achievement ratio of 80% or higher, and to maintain the ratio of required reblowing at 10% or lower.